Unusual Coastal Breeding in the Desert-Nesting GrayGull (Leucophaeus modestus) in Northern Chile

Authors: Aguilar, Roberto, Simeone, Alejandro, Rottmann, Jürgen,Perucci, Marietta, and Luna-Jorquera, Guillermo

Source: Waterbirds, 39(1) : 69-73Published By: The Waterbird Society

URL: https://doi.org/10.1675/063.039.0108

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69

Unusual Coastal Breeding in the Desert-nesting Gray Gull (Leucophaeus modestus) in Northern Chile

Roberto Aguilar1, Alejandro Simeone2,*, Jürgen Rottmann3, Marietta Perucci1 and Guillermo Luna-Jorquera4

1Corporación de Estudios Culturales y Ambientales (CUCTAM), Antilhue 01837- 6, Antofagasta, Chile

2Departamento de Ecología y Biodiversidad, Facultad de Ecología y Recursos Naturales, Universidad Andrés Bello, República 440, Santiago, Chile

3Fundación para la Sustentabilidad del Gaviotín Chico, Ongolmo 965, Mejillones, Antofagasta Chile

4Universidad Católica del Norte, Millennium Nucleus Ecology and Sustainable Management of Oceanic Island, Centro de Estudios Avanzados en Zonas Áridas, Larrondo 1281, Coquimbo, Chile

*Corresponding author; E-mail: asimeone@unab.cl

Abstract.—The Gray Gull (Leucophaeus modestus) has the unique habit among gulls of nesting in the interior Atacama Desert, up to 100 km from the coast. During the 2014-2015 austral breeding season, two breeding colo-nies were recorded on the coast within 90 m of the shoreline in the Antofagasta Region, northern Chile. The new colonies ranged in size from 40 (Playa Grande) to 150 (Playa Brava) nests. Egg laying was synchronous in both colonies and most likely occurred in late November 2014, coinciding with egg laying in desert colonies. The colony at Playa Brava was successful, but the one at Playa Grande was deserted due to feral dog (Canis familiaris) attacks. The habitat used by Gray Gulls resembled that reported for desert colonies, with flat plains covered with small rocks, which provide protection to chicks from intense solar radiation. This unusual coastal nesting behavior could result in the modification of certain life history and behavioral traits in the Gray Gull (e.g., chick growth rates, en-ergy expenditure, and foraging ranges), which have evolved to breed in severe desert conditions. We suggest that coastal breeding is adopted by Gray Gulls during El Niño years in response to reduced food supply. During El Niño years, Gray Gulls would move to the coast where access to food is better and thermoregulatory costs are lower, but predation is higher. During non-El Niño years, Gray Gulls would resume their ancestral desert-nesting strategy in which traveling distances between the coast and nesting grounds are considerable and thermoregulatory costs are higher, but predation risks are lower. Future observations should confirm if Gray Gulls continue breeding at coastal sites during El Niño years or if this becomes a regular behavior independent of oceanographic conditions. Received 7 April 2015, accepted 12 May 2015.

Key words.—breeding biology, coastal nesting, desert nesting, El Niño, Gray Gull, Leucophaeus modestus, north-ern Chile.

Waterbirds 39(1): 69-73, 2016

Formerly included within the genus Larus (Remsen et al. 2015), the Gray Gull (Leuco-phaeus modestus) is a seabird endemic to west-ern South America, ranging along the coast from Ecuador to southern Chile (Howell and Dunn 2007). While it forages exclusively in coastal habitats (Howell et al. 1974; Ryan et al. 1987), the nesting of this species has been confined exclusively to the barren Atacama Desert at distances up to 100 km from the nearest coastline (Goodall et al. 1945; Howell et al. 1974). This habit is unique among Lari-dae (Howell and Dunn 2007), and the Gray Gull is probably one of the few bird species capable of nesting in the interior Atacama Desert, the driest place in the world (Mof-fett 1969). Explanations for the evolution of the desert-nesting habits of the Gray Gull are

uncertain. Howell et al. (1974) proposed that Gray Gulls may have gradually reached their current nesting sites in the barren desert in an attempt to avoid the more predator-occu-pied coast. Another hypothesis proposed by these authors suggests that the species moved inland to nest around inland lakes during the Pleistocene; this energetically expensive pat-tern may have persisted due to the scarcity of predators in the desert.

Since the first colonies were reported by Goodall et al. (1945), 11 nesting sites for this species have been reported in the literature, all located in the desert at distances between 25 and 100 km from the nearest coast and ranging in size between 50 and 12,000 pairs (Guerra et al. 1988b; Aguilar et al. 2013; Fig. 1). Most recently, however, a new colony

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70 Waterbirds

was discovered at Quillagua (Fig. 1), and a preliminary estimate suggests that it may contain up to 30,000 pairs (R. A. unpubl. data). Gray Gulls are known to shift nesting sites from one location to another within a few decades (Aguilar et al. 2013). The rea-sons for such colony dynamics are not well understood, and explanations may include natural predation and human perturbation associated with mining and vehicle raids (Aguilar et al. 2013). In this note, we report the unusual establishment of two nesting sites of Gray Gulls on the coast of northern Chile.

Methods

In early January 2015, local residents reported two new colonies of Peruvian Terns (Sternula lorata) at local beaches north of Antofagasta in northern Chile (Fig. 1). Upon survey on 5 and 6 January, it was determined that two colonies of Gray Gulls (not Peruvian Terns) were actually present.

The first colony was located at Playa Brava (22° 57′ S, 70° 18′ W) within 20-90 m of the coastline and 78 km north of Antofagasta (Fig. 1), and extended over a flat plain with sandy substrate covered with small rocks (Fig. 2). This colony was visited weekly until late February (mainly between 10:00 and 12:00 hr). During each visit, we counted the adults, chicks and fledglings at the colony with 10x42 binoculars and a 60x spotting scope. The second colony was situ-ated at Playa Grande (23° 01′ S, 70° 20′ W) within 50-60 m of the coastline and 9 km south of Playa Brava (Fig. 1). Habitat was similar to that described for the Playa Brava colony.

Results and Discussion

At the time of the first visit on 5 January 2015, the Playa Brava colony contained ap-proximately 150 nests: 74% contained only eggs, 6% contained eggs and chicks and 20% contained only small (< 1 week old) chicks (Fig. 2). By late February, this colony produced 107 fledglings (Fig. 3).

Figure 1. Location of Gray Gull colonies in northern Chile. Open circles indicate colonies active as of January 2015, filled circles indicate colonies with no recent activity. Open triangles indicate the two new coastal breeding sites described in this study.

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Coastal Breeding in Gray Gulls 71

The Playa Grande colony was inspected on 6 January 2015 and included 40 nests: 93% contained only eggs, 3% contained

eggs and chicks and 4% contained small chicks. On the following day, this colony was totally deserted and footprints of feral dogs

Figure 2. Adult Gray Gull shading a chick at the coastal colony of Playa Brava, northern Chile. Photograph by R. Aguilar.

Figure 3. Counts of Gray Gulls (adults, chicks and fledglings) at the coastal colony of Playa Brava between January and February 2015.

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72 Waterbirds

(Canis familiaris) were present. Turkey Vul-tures (Cathartes aura) were scavenging on the recently abandoned eggs. No further monitoring of this colony was performed.

Breeding activity at both colonies seemed to be rather synchronous, although Playa Brava contained a higher proportion of nests with chicks, suggesting an earlier start. Judg-ing by the size of the chicks at the time of the first visits (see Aguilar et al. 2013 and Guerra et al. 1988a for chick aging) and assuming an incubation period of 29-30 days (Howell et al. 1974; Aguilar et al. 1994), first eggs were most likely laid between the second and third week of November 2014. Aguilar et al. (2013) reported that in desert colonies Gray Gulls usually lay eggs from early November throughout December, so timing for both coastal sites seems to be aligned with desert colonies.

Nests were located in close proximity to small rocks, a strategy that has been de-scribed as crucial for chick survival in desert colonies as they provide protection against the intense solar radiation during the day and low temperatures and wind during the night (Guerra et al. 1988b). Although ambi-ent temperatures during the day are consid-erably lower along the coast than in the bar-ren desert (Luna-Jorquera et al. 2003), Gray Gulls seem to still seek this habitat feature in coastal colonies.

Despite regular scavenging by Turkey Vultures and foxes (Lycalopex spp.), it is usual to find abundant chick carcasses and egg shells from previous breeding seasons at Gray Gull colonies in the desert (R. Aguilar, pers. obs.). At Playa Brava, however, no old carcasses were found and only a few recently dead chicks and fledgling carcasses were present. This strongly suggests that no oc-cupation has occurred in previous years at this site and that Gray Gulls bred for the first time at this locality during the 2014-2015 sea-son. Additionally, Playa Brava was regularly visited in previous years for seabird counts and the presence of a Gray Gull colony was never detected.

This unusual coastal nesting behavior could result in the modification to some ex-tent of certain life history traits in the Gray

Gull, which has evolved several metabolic and behavioral strategies to breed in severe desert conditions that include high ambient temperatures during the day, low tempera-tures during the night, low air humidity and long distances to the foraging areas (Luna-Jorquera et al. 2003). For instance, Gray Gull chicks grow at reduced rates compared to other gull species, which is most likely to cope with the long foraging distances that limit chicks to a single daily meal (Guerra et al. 1988a); chicks from Playa Brava prob-ably receive several meals during the day and this would likely increase their growth rates. Gray Gull embryos tolerate temperatures be-tween 17 and 42 °C, an adaptation likely to overcome the strong thermal amplitude ob-served in the desert (Aguilar et al. 1998); it is conceivable that embryos in coastal colonies would have lower energy expenditure. Simi-larly, it is expected that incubating adults would dedicate less time in thermoregula-tion behaviors and thus have lower energy expenditure. On the other hand, these com-parative advantages could be counteracted by a strong predation pressure along the coast as was observed at the Playa Grande colony.

While this is the first time that Gray Gulls have been reported to breed in coastal north-ern Chile, Zavalaga et al. (2008) mentioned “thousands” of Gray Gulls nesting at El Can-grejal beach along the Peruvian coast during the 1999-2000 austral breeding season. This colony, however, had very low success (only 60 fledglings) potentially due to intense pre-dation by foxes and local fishermen. That same season, nearly 500 pairs attempted to nest at Independencia Island (also known as La Vieja Island) where birds were observed establishing nests and copulating, but no eggs were laid (I. García-Godos, pers. com-mun.). Together, the evidence for Chile and Peru suggests that Gray Gulls are not re-stricted to breeding in the interior Atacama Desert as previously thought, but that under certain circumstances they also are able to successfully breed in coastal areas.

It is interesting to note that coastal breed-ing in Peru occurred shortly after the 1997-1998 El Niño event and the coastal breeding

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Coastal Breeding in Gray Gulls 73

reported in this note occurred under a de-veloping El Niño event. It is well known that during El Niño Gray Gull colonies in the desert suffer complete desertion (Guerra et al. 1988c), so it may be that during El Niño events some Gray Gulls abandon the desert-nesting strategy and shift to coastal breed-ing, at least temporarily. If this hypothesis is correct, during El Niño Gray Gulls would move to the coast where access to food is better and thermoregulatory costs are lower; these benefits, however, would be counter-acted by higher predation risks (Zavalaga et al. 2008). During non-El Niño years, Gray Gulls would resume their ancestral desert-nesting strategy in which traveling distances between food sources and nesting sites are considerable and thermoregulatory costs are higher, but predation risks are compar-atively much lower and restricted to native predators. Future field observations should confirm if Gray Gulls continue breeding at coastal sites during El Niño events or if this becomes a regular behavior independent of the prevailing oceanographic conditions.

Acknowledgments

Observations were conducted within the activities of the project “Breeding of the Gray Gull in northern Chile” granted to CULTAM. Financial support for this project was provided by Minera Escondida Limitada, Minera Antucoya and Minera Sierra Gorda SCM through Act D. L. N° 3063 of donations for scientific purposes. The following people provided help during the field work: Alberto Rivera, Sylvia Hernández, Bárbara Olmedo, Román Figueroa and Margarita Fernández (Fundación para la Sustentabilidad del Gaviotín Chico), Arami Silva and Patricia Bolados (CULTAM), Camila Ardiles (U. An-drés Bello) and Diego Valverde (U. Católica del Norte). Carlos Zavalaga and Ignacio García-Godos shared their unpublished observation on nesting Gray Gulls in coastal Peru. Cristóbal Anguita helped us with figure produc-tion. We are very grateful to all of them.

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